VCSELs are widely used as light sources for optical interconnect devices, storage area networks, and sensors. The most common configuration of a VCSEL is a two-terminal VCSEL that includes a conducting n-type substrate, an n-type distributed Brag reflector (DBR) disposed on the top surface of the substrate, an intrinsic layer (active region) disposed on top of n-type DBR, a p-type DBR disposed on top of the intrinsic layer, an ohmic n-contact disposed on the bottom surface of the n-type substrate, and an ohmic p-contact disposed on the top surface of the p-type DBR. The ohmic n- and p-contacts correspond to respective first and second terminals of the VCSEL.
When an electric potential is applied across the terminals, electrons from the n-type layers that are adjacent the intrinsic layer and holes from the p-type layers that are adjacent the intrinsic layer are injected into the active region of the intrinsic layer where they combine to produce photons. This combining of holes and electrons in the active region to produce photons is a phenomenon known as spontaneous emission. As the photons pass out of the active region, they are repeatedly reflected by the DBRs back into the active region, which results in more recombination of electrons and holes in the active region. This is a phenomenon known as stimulated emission. The repeated reflection of photons by the DBRs back into the active region provides the “pumping” action that leads to lasing.
In such VCSELs, the front, or active, side of the VCSEL is the side of the p-type DBR on which a ring-shaped ohmic p-contact is disposed and the back side of the VCSEL is the bottom surface of the substrate. These contacts are typically created by depositing several thin metal layers on the surface of the p-type DBR and the substrate. The art of selecting the appropriate metal and subsequence processing required to form the ohmic contact is well known. The laser light propagates out of the VCSEL through the top surface of the p-type DBR through an aperture defined by the shape of the ohmic p-contact. Energy supplied to the VCSEL that is not converted into light is dissipated as heat.
In a high-power VCSEL, in order to deliver maximum power in the smallest possible chip area, either a single VCSEL aperture of very large diameter or an array of many VCSEL apertures located in close proximity to one another within a single die is employed. Such a high power configuration means that heat dissipation is concentrated in a small area. Therefore, there is a need to spread out the dissipated heat to prevent the temperatures of the VCSELs from rising to the point that their performance is adversely affected. One way to provide such a highly thermally-conductive path is to replace the ring-shaped front-side (the side on which the ohmic p-contacts are disposed in the common VCSEL configuration described above) metal contact with a disk shape metal contact with no opening, and to attach the front-side of this metal contact to a substrate that is more thermally conductive than the native substrate on which the VCSEL is formed.
The attachment is accomplished using one of a plurality of known wafer-to-wafer bonding techniques. Such bonding techniques result in the metal layer that is used to form the ohmic p-contacts of the VCSEL being in contact with the thermally-conductive substrate. The highly thermally conductive substrate is used as a support to process the bonded wafer. The completed VCSEL die, after singulation from the wafer, is attached to an assembly with the thermally-conductive substrate in direct contact with a heat sink of the assembly. Heat generated in the active region of the VCSEL spreads into a larger area in the thermally-conductive substrate at a much shorter distance than a path through the native substrate of lower thermal conductivity. The overall thermal resistance of the path from the active region of the VCSEL to the heat sink of the assembly is much lower than that of a path that includes the native substrate in direct contact with the heat sink of the assembly.
One of the problems with this type of wafer-to-wafer bonded arrangement is that interfacial stress at the bonding interface can lead to bonding failure between the VCSEL wafer and the heat-dissipating wafer. For example, stress due to the different coefficients of thermal expansion (CTEs) of the VCSEL wafer and of the thermally-conductive wafer can lead to bonding failure. In addition, particles trapped between the wafers can create bonding failures. Moreover, the active side of the VCSEL wafer is typically covered with a nitride layer in which holes are etched and into which the ohmic p-contact metal is evaporated. The shapes of the holes in the nitride layer are transferred to the p-contact metal layer. These holes can create bonding voids that impede heat flow.
Another problem is that, since the contiguous metal coverage of the front-side of the aperture needed to achieve low thermal resistance blocks light emission from the front-side, light must exit the VCSEL from the back side. However, a VCSEL operating at a wavelength that is shorter than the absorbing threshold of the native substrate cannot emit light from the back-side with the native substrate in place.
Accordingly, a need exists for a wafer-to-wafer bonding solution that reduces the likelihood that a wafer-to-wafer bonding failure will occur and that improves heat flow from the VCSEL to the heat sink of the VCSEL assembly. A need also exists for a back-side-emitting VCSEL wafer bonded to a highly thermally-conductive wafer and having the native substrate of the VCSEL wafer removed.